This application claims the priority benefit of Taiwan application serial no. 88110533, filed Jun. 23, 1999, the full disclosure of which is incorporated herein by reference.
1. Field of Invention
The present invention relates to a structure of a printed circuit board. More particularly, the present invention relates to a structure of a printed circuit board with a stacked daughter board.
2. Description of Related Art
After devices are packaged, the devices are mounted on a substrate such as a printed circuit board for coupling with other devices; the most common application is a mainboard or motherboard.
FIG. 1 is a schematic diagram of a conventional mainboard.
Referring to FIG. 1, devices such as a microprocessor 12, a chip 14 and a slot 16 are mounted on a mainboard 10 that is usually a four-layer printed circuit board. The microprocessor 12 is used to run programs and the chip 14 is a communication bridge between the microprocessor 12 and the slot 16 or the microprocessor 12 and other devices. The slot 16 is for an expansion card such as a network card or a sound card. The devices are coupled with each other by traces 18 for transmitting data.
Since the amount of data transmitted between each device is different, the number of traces 18 between any two devices is also different. This means that the distribution of the trace density on the mainboard 10 is not even. For example, the trace density of trace region 20 between the microprocessor 12 and the chip 14 is high, because most data are transmitted between the microprocessor 12 and the chip 14. In contrast, the trace density in trace region 22 between the chip 14 and the slot 16 is low, because fewer data are transmitted between the chip 14 and the slot 16.
With the increasing power of the microprocessor and the increasing number of devices, the number of traces on the mainboard must increase to deal with more data transmitted between devices. This means that the distribution of the trace density on the mainboard becomes worse; the high trace density region becomes denser. However, the area of the mainboard for forming the traces is fixed, therefore the trace density is limited. When the trace density exceeds the limit, the layout of the traces cannot be formed on the general four-layer printed circuit board, and a printed circuit board having more layers is needed. However, dense traces are only formed in some regions and the cost of the printed circuit board having more layers is high, so the cost of achieving the high density trace layout is increased by using a printed circuit board having more layers.
The invention provides a structure of a printed circuit board with stacked daughter board. The structure of the printed circuit board is a daughter board stacked on a high trace density region of a mainboard or motherboard, so that a high-density trace layout is achieved. Additionally, by using the daughter board, the trace layout is flexible because a bridge circuit is formed by the daughter board, so that circuits on the motherboard can be changed by changing the daughter board. Moreover, design of the printed circuit board is modularized by using the daughter board.
As embodied and broadly described herein, the invention provides a structure of a printed circuit board with stacked daughter board. The structure includes a motherboard and at least a daughter board. The motherboard comprises two signal layers plus a power layer, and a ground layer. Isolation layers separate each of the layers. The signal layers serve as surfaces of the motherboard and contacts are formed on one of the signal layers. The daughter board also comprises two signal layers plus a power layer, and a ground layer. Isolation layers separate each of the layers. The power layer or the ground layer serves as a surface of the daughter board and contacts are formed on the surface. The surface of the daughter board formed by the ground layer or the power layer is stacked on the motherboard and the daughter board contacts are coupled with the motherboard contacts.
In another embodiment grooves are formed on opposite sides of the daughter board. The inner surface of each groove and a region around each groove are covered with a conductive layer. The surface of the daughter board formed by the ground layer or the power layer is stacked on the motherboard. The grooves on the daughter board are coupled with the contacts on the motherboard.
The daughter board is stacked on the motherboard so the high-density trace layout can be achieved for the motherboard. Furthermore, the design of the printed circuit board assembly made from the motherboard and the daughter board becomes flexible and modularized because the function of the printed circuit board is easily changed by changing the daughter board.
It is to be understood that both the foregoing general description and the following detailed description are exemplary, and are intended to provide further explanation of the invention as claimed.